This invention relates to curable unsaturated polyester resin compositions.
Fiber-reinforced plastics (FRP) have superior characteristics not only regarding mechanical strength, and heat, water and chemical resistance but also regarding productivity, and are used extensively for bathtubs, water tank panels and bathroom sinks. Recently, unsaturated polyester sheet molding compounds (SMC) and bulk molding compounds (BMC) are coming to be recognized as excellent plastic materials for automotive exterior body panels and are applied not only to exterior parts such as spoilers, air-intakes and rocker panels but also to main exterior panels such as engine hoods, roofs and trunk lids.
It is known, however, that thermosetting unsaturated polyester resins shrink significantly as they cure. For this reason, molded products of such resins tend to crack or warp. Glass fibers used as reinforcement tend to become easily visible and pinholes (porosity) appear to adversely affect the surface appearance and paintability. It is also a problem that they have inferior dimensional stability.
In view of the above, it has been known to mix thermoplastic resins such as polystyrene, polyvinylacetate, polymethyl methacrylate, and block copolymers of a conjugated diene monomer and an aromatic vinyl monomer as low profile additives to unsaturated polyester resins (29th National SAMPE Symposium, Apr. 3-5, 1984). Block copolymers of lactone and conjugated diene (U.S. Pat. Nos. 4,287,313 and 4,360,643) for improving compatibility and unsaturated polyester resin compositions with liquid polymer tougheners introduced into unsaturated matrix (U.S. Pat. No. 4,290,939 and Canadian Patent 1,152,682) have also been disclosed. There is yet to be discovered, however, any thermoplastic resin which is satisfactory from all points of view such as compatibility and dispersibility when mixed with unsaturated polyester resins and ability to reduce shrinkage and surface smoothness of products. At the present time, therefore, they are being used only for some, and not all, of these required characteristics. Since molding compounds applied to automotive exterior body panels, in particular, are required to give surface qualities comparable to those of a steel sheet, unsaturated polyester resin compounds containing known thermoplastic resins as low profile additives are hardly satisfactory.
The present inventors have already disclosed some curable unsaturated polyester resin compositions in view of the above (Japanese Patent Publications Tokkai 62-195041, 63-3056 and 63-6048 and U.S. patent application Ser. No. 013,905). Although these compositions have low shrinkage and improved strength, there still remain problems regarding surface smoothness and paintability.